It was getting pretty dark by the time I finished, however, I managed to get the engine into the frame for a test fit and to see what sort of space I have.

Pictures below:

Now, I think I wont be able to get away with this build without modfying the frame. However, the gearbox should fit with the engine in its current location. Of course the flywheel wont fit with the engine in its current location as its goes where the frame down tube is.